Polyimides are synthetic organic resins characterized by repeating imide linkages in the polymer chain which may or may not be end-capped with polymerizable or inert (i.e., non-polymerizable) chemical groups. Polyimides exist in both linear and cross linked forms and are noted for their outstanding chemical and physical properties, particularly their high temperature oxidative stability and strength. In addition to their use as adhesives and molded articles, polyimides may be used as precured films and fibers, curable enamels, laminating resins, and as matrices for fiber reinforced composites.
The objective in polyimide structure-property-process engineering has been to obtain materials which exhibit high temperature stability; strength or toughness, and processability. Where high use temperature is required, the polyimide must have a high glass transition temperature as well as sufficient thermo-oxidative stability below its glass transition temperature. While many aromatic polyimides have some of the highest use temperatures, previous polyimides generally do not possess all three desired properties. In particular, recent advances in composite fabrication processes (e.g. liquid molding processes such as resin transfer molding (RTM) and resin infusion (RI)), has resulted in the need for polyimides that retain the excellent thermal and mechanical characteristics of traditional polyimides but exhibit significantly lower melt viscosities. The materials described in this invention possess high glass transition temperatures, excellent mechanical properties, excellent thermo-oxidative stability, and are suitable for RTM and resin infusion composite fabrication procedures.
Unlike autoclave fabrication procedures traditionally used to fabricate polyimide composite components, RTM and RI are essentially solvent-free processes which impart significant cost advantages and component design flexibility. RTM and RI enable fabrication of highly complex shapes that would otherwise be extremely difficult or not possible using solvent based prepreg or tape/tow placement techniques. Typically, these melt infusion processes involve the placement of a textile preform or mat (e.g. carbon, glass, ceramic, etc.) in a mold cavity which is subsequently injected or infused with liquid resin at an elevated temperature, usually under reduced pressure. The molten resin permeates through the woven preform to completely wet out the preform, followed by increasing the temperature to chemically crosslink or “cure” the thermoset resin. During this step, external hydrostatic pressure is often applied to the mold to ensure consolidation. In this process it is important that no volatiles are present (as residual solvent, chemical defects, or residual condensation reactions) since these volatiles will result in composite voids which ultimately reduce the properties of the finished component. These features are generally difficult to achieve with polyimide resins. Commercial resins, including vinyl esters, epoxies, and bismaleimides are available that are processable by RI and/or RTM but are limited in use temperature relative to the aromatic imide based materials described in this patent.
Oligomeric addition polyimides comprised of aromatic dianhydrides, aromatic diamines, and terminated with aromatic end-capping agents are generally described in prior patent literature (U.S. Pat. No. 5,138,028). What is described in this present patent is a preferred composition for an imide oligomer that exhibits a number of novel and advantageous processing characteristics while maintaining good thermal and mechanical properties. This present invention exhibits unanticipated improvements in properties and processing characteristics, as compared to state-of-the-art polyimides, that are particularly advantageous in the manufacture and use of fiber reinforced composites materials manufactured by liquid molding processes.
U.S. Pat. No. 6,359,107 discloses low molecular weight imide oligomers suitable for RTM and RI processes based on a flexible diamine (selected from the group consisting of: 1,3-bis(3-aminophenoxy)benzene and 1,3-bis(4-aminophenoxy)benzene) and a rigid diamine (selected from the group consisting of: 1,3-diaminobenzene, 9,9′-bis(4-aminophenyl)fluorine, and 3,4′-diaminodiphenyl ether). Using a phenylethynyl functional reactive terminal group, said combination of diamines, and an aromatic dianhydride selected from the group consisting of: 3,3′,4,4′-biphenyltetracarboxylic dianhydride, 4,4′-oxydiphthalic anhydride, pyromellitic dianhydride, and 4,4′-biphenoxydiphthalic anhydride, the inventors were able to achieve imide oligomer compositions with low melt viscosity (described as less than 60 Poise at temperatures less than 300° C.) and high glass transition temperatures (>250° C.). To achieve these values required very low oligomeric molecular weights (stoichiometric ratio of 0.19 to 0.72 for dianhydride to diamine), concentration of the rigid diamine of less than 50% (molar) of the total diamines, and cure temperatures greater than approximately 350° C.
U.S. Pat. No. 6,184,333 discloses low toxicity high temperature thermoset polyimides that utilize a diamine selected from the group consisting of 4,4′-[1,4-phenylene-bis(1-methylethylidene)] bisaniline and 4,4′-[1,3-phenylene-bis(1-methylethylidene)] bisaniline. These thermoset polyimides and imide oligomers do not use a diaryl substituted acetylene as an end-capping agent.
As the artisan will appreciate, there remains a need for improved imide oligomers that can be used to fabricate composite materials with an overall combined improvement in toughness, mechanical performance, thermo-oxidative stability, higher glass transition temperature, lower cost, and lower cure temperature than state-of-the-art melt processable imide oligomers.